ci/update-maintainers: drop issues:write permission

We requested this permission to be able to create new labels, when
labelling the PR. However that is not necessary here, because a) we
don't apply any labels and b) if we did we could create them manually.

Our app installation doesn't have `issues:write`, so requesting it was
causing the workflow to fail.

.github/workflows/update-maintainers.yml

@@ -24,11 +24,9 @@ jobs:
     # Permissions required for workflow
     # `contents`: to update maintainers file
     # `pull-requests`: to create pr
-    # `issues`: to label pr
     permissions:
       contents: write
       pull-requests: write
-      issues: write
     env:
       pr_branch: update/maintainers-${{ github.ref_name }}
     steps:
@@ -41,7 +39,6 @@ jobs:
           private-key: ${{ secrets.CI_APP_PRIVATE_KEY }}
           permission-contents: write
           permission-pull-requests: write
-          permission-issues: write
 
       - name: Get GitHub App user info
         id: user-info